A population of individuals potentially at risk for psychiatric disorders was identified by screening 375 college student volunteers for low platelet monoamine oxidase (MAO) activity Levels. The lower and upper 10 percent in MAO activity were interviewed and average evoked response, personality, perceptual and family history data obtained. Low MAO probands reported more frequent psychiatric or psychological counseling and problems with the law. Families of low MAO probands had an eight-fold increase in suicide incidence over those of high MAO probands. Both in these low MAO probands and in NIH psychiatric inpatients the combination of low MAO and average evoked potential augmenting was associated with increased frequency of suicide or suicide attempts. This suggests that reduced MAO levels, reported previously in patients with affective disorders and chronic schizophrenia, may predict a vulnerability to psychiatric disorder.